Thermoelectric element

ABSTRACT

A thermoelectric element includes at least one thermopair and a pn-junction. The thermopair has a first material with a positive Seebeck coefficient and a second material with a negative Seebeck coefficient. The first material is selectively contacted by way of a conductor with the p-side of the pn-junction, and the second material is selectively contacted by way of a conductor with the n-side of the pn-junction.

BACKGROUND OF THE INVENTION

The present invention concerns a thermoelectric element comprising atleast one thermopair and a pn-junction, wherein the thermopair has afirst material with a positive Seebeck coefficient and a second materialwith a negative Seebeck coefficient and a thermoelectric generator and athermoelectric cooler with a thermoelectric element of that kind.

The state of the art is divided into various areas which involvedifferent development stages.

The thermoelectric effect has already been known for more than 100years, and there is a wide range of materials which can be used for thedirect conversion of a temperature difference into electric current(thermoelectric generator) or for cooling upon the application of anexternal voltage source (thermoelectric cooler). Technicalimplementation of the generator effect has hitherto always been based ona common fundamental structure (FIG. 6). Two different metals or twodifferently doped (n- and p-doped) semiconductors are connected to anend, in the normal case the hot end, by way of a metallic conductor, andthen the current can be taken off at the other end, which is normallythe cold end. The energy for overcoming the differences in theelectrical potential between the materials at the contact locations istaken from the ambient heat (Peltier effect).

In order to achieve conversion of the temperature gradient into electriccurrent in the most effective possible fashion, the thermoelectricelements are assembled to constitute a module in such a way that theindividual elements are connected electrically in series but thermallyin parallel. Those modules can in turn be combined to form larger units(FIG. 7). Such an apparatus can be found for example in EP 339 715 A1.

The choice of the materials used is based on achieving the maximumpossible efficiency in the desired temperature range. The efficiency isgenerally characterized by figure of merit Z=S²/ρκ (S is the Seebeckcoefficient or absolute differential thermoelectric force, ρ denotes thespecific resistance and κ denotes thermal conductivity). High efficiencyis achieved in a material with a high Seebeck coefficient, a lowspecific resistance, and a low thermal conductivity.

The thermoelectric elements based on pairs of n- and p-type materialblocks are those which in that respect are most developed, but havescarcely any advances to show even after more than 50 years ofdevelopment time. Those thermoelectric elements can be obtained as aproduct for decades and are used in particular for cooling(thermoelectric cooler, Peltier module).

The essential advantage of that state of the art is that the productionprocesses have been known for decades and are in a mature state.However, it also has the following disadvantages:

The properties of a material which are important for thermoelectricity(S . . . Seebeck coefficient, ρ . . . specific resistance and κ . . .thermal conductivity) can be influenced independently of each other onlyto a very slight degree. That relationship limits the efficienciesachievable at the present time to about 10-20% of the Carnot efficiency.

The configuration of the temperature gradient has no influence onefficiency as it is only the overall difference in temperatures betweenthe hot side and the cold side that plays a part in conventionalthermoelectric elements by virtue of the linear relationship between thethermoelectric force and the temperature difference.

Power density is too low to economically provide high power levels.

A further highly promising line of development in the boundary area ofthermoelectricity and thermoionic effect has been followed by Eneco,Inc. of Salt Lake City, Utah, USA (Yan R. Kucherov and Peter L.Hagelstein), with the development of a thermoionic converter and athermal diode.

A thermoionic converter (FIG. 8) comprises a heated metal plate and acooled metal plate separated by a vacuum and an external circuit. Due tothe higher temperature in the heated metal plate, more electrons haveenough energy to overcome the potential barrier in the direction of themetal plate than in the reverse direction. In that way, current can beobtained from a temperature difference. It will be noted however thatthis process can only take place at very high temperatures because ofthe high potential barrier.

Thermal diodes have the same functional components, but the vacuum isreplaced by a semiconductor. A diagrammatic structure of an n-typethermal diode from Eneco, Inc. is shown in FIG. 9. The semiconductor inplace of the vacuum provides for a lower potential barrier and thereforethe thermal diode functions even at lower temperatures.

With the correct arrangement of further potential barriers between thecollector and the gap semiconductor, that arrangement prevents electronsfrom flowing back again. Accordingly, electrons are accumulated and ahigher working voltage can be attained.

The advantages of the above-described configuration are as follows:

The efficiency of conversion of heat into current is higher.

In contrast to thermoelectric elements of the general kind set forth,thermal diodes can be connected in series without efficiency sufferingtherefrom. Accordingly it is easier to reach the maximum theoreticalefficiency.

However, the disadvantages of this configuration are as follows:

This structure operates only with electrons, there is no thermal diodefor holes, therefore the circuit has to be closed by way of anelectrical conductor, by way of which heat also flows and thereforereduces efficiency.

The effect used only occurs if the thicknesses of the barriers are inthe region of the leakage lengths and thus at some 100 nanometers (whenInSb is used 1.5 micrometers). At higher temperatures the diffusion ofmaterials is greater, therefore the potential barriers are rounded withtime and the lengths necessary for maintaining the effect are no longerobserved. The temperatures which can be used for current generation aretherefore severely limited upwardly.

In order to utilize the generation of electron-hole pairs, a pn-junctionwith a temperature gradient can be utilized in known thermoelectricelements (AT 410 492 B).

In the structure shown in FIG. 10, electron-hole pairs are generated atthe hot end as the thermal equilibrium between generation andrecombination is shifted to the benefit of generation due to the chargecarrier drift because of the temperature gradient. The pn-junction is apart of the overall structure which structurally cannot be separatedfrom the location of the temperature gradient.

The advantages of the above-described configuration are as follows:

The operating temperatures can be extremely high depending on therespective material used.

Simple structure similar to a solar cell.

However, the disadvantages of this configuration are as follows:

The recombination of charge carriers cannot be entirely prevented sothat efficiency is reduced.

Thick layers are necessary for the transport of charge carriers, andthey make more demanding manufacturing methods necessary.

DE 101 36 667 A1 discloses the integration of a diode into an arm of aPeltier element.

GB 953 339 A1 discloses the structural combination of a Peltier elementwith a diode.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved thermoelectricelement.

That object is attained by a thermoelectric element having the featuresof the present invention.

The basically novel idea is that of using a pn-junction (for example adiode) as a generation center of electrons and holes as an expansion ofa known thermopair.

Without the generation of electron-hole pairs, a thermopair acts only asa charge carrier pump, the properties of which are determined by thechoice of a material. The dimensions or other external properties havean only limited influence. Those limits are circumvented by thegeneration of new electron-hole pairs and markedly higher levels ofefficiency are accordingly possible.

When using the thermoelectric element according to the invention in athermoelectric generator having the features of the present invention,the idea of the invention is that the thermoelectric voltage of athermopair, which is caused by a temperature difference, is used to biasa pn-junction electrically connected to the hot side in the reversedirection (blocking direction). Electron-hole pairs are generated inthat pn-junction by thermal excitation. The extent of that thermalexcitation depends on the temperature, the band gap and the number ofgeneration centers:

$I_{electric} \approx G \approx {N_{t}{\exp\left( {- \frac{E_{t} - E_{i}}{kT}} \right)}}$

In a pn-junction with bias in the reverse direction current flows byvirtue of the generation of electron-hole pairs which are immediatelyseparated by the built-in electrical field and therefore appear as a netcurrent.

The advantages of the present invention are as follows:

More charge carriers can be produced by the generation of electron-holepairs, than are predetermined by the doping. That makes it possible toachieve a higher power density and efficiency.

The generation of electron-hole pairs is spatially separated from thelocation at which the thermoelectric voltage occurs and can therefore beoptimized by the choice of material or another production process,independently of heat conduction. Also, no temperature gradient isnecessary.

Due to the use of a material with a lower band gap in comparison withthe thermopair, both transport and also generation of the chargecarriers can be optimized.

Further advantageous embodiments of the invention are described below.

For example, the conductor selectively contacting the first materialwith the p-side of the pn-junction and the conductor selectivelycontacting the second material with the n-side of the pn-junction can bemetallic conductors.

The thermopair and the diode can be connected spatially separately fromeach other only by an electrical conductor. The thermopair and the diodecan also be of different materials in order to be able to optimize thegeneration of charge carriers and transport thereof separately from eachother. For thermal generation both the number of generation centers andalso the relationship of thermal energy (temperature) and energy gap aredecisive. Transport of the charge carriers in the thermopair is to beoptimized separately, for example by using different materials,manufacturing methods and material structures (nanotechnology formodulation of thermal conductivity by supergratings, quantum dots,etc.).

For optimizing the current and thus charge carrier generation, theintrinsic concentration n_(i) (thus temperature and band gap) and thegeneration life τ_(g) (thus the number of generation centers, theeffective cross-section and the temperature) are the determiningphysical parameters:

${I \approx G} = \frac{n_{i}}{\tau_{g}}$$n_{i} = {\sqrt{N_{C}N_{V}} \cdot {\mathbb{e}}^{({- \frac{E_{g}}{2{kT}}})}}$$\tau_{g} = \frac{2{\cosh\left( \frac{E_{T} - E_{i}}{kT} \right)}}{v_{th}\sigma_{0}N_{T}}$

For the invention, the same physical parameters are of significance asin the state of the art. However, it will be noted that with theinvention the dependencies of the parameters among each other can beweakened as the lateral change in the parameters has an influence on theefficiency of the overall structure.

In addition, the invention affords a wealth of new candidates for goodthermoelectric materials.

The preferred materials for carrying out the invention aresemiconductors, wherein the temperature range used determines the choiceof the materials.

Compound semiconductors, by virtue of the low thermal conductivity, arethe preferred materials for thermoelectric modules. Some examples ofgood thermoelectric materials are: Bi₂Te₃, PbTe, SiGe, clathrate andmaterials involving a perovskite structure and semiconducting polymers.For making the pn-junction, the materials can be used without havingregard to low thermal conductivity as no temperature gradient isnecessary.

High to very high doping levels are preferred to keep the specificresistance as low as possible. It will be appreciated that the valuesfor the dopings are dependent on the material. As an example: with PbTedoping levels of 10¹⁸ cm⁻³ and higher are necessary.

There are in particular three large areas in which the invention can beemployed:

1. Thermoelectric generators for direct conversion of a temperaturedifference into current. With that concept any residual heat involved,which otherwise would remain unused, can be used.

2. Thermoelectric cooler: one end becomes hot and the other cold due toa flow of current. That effect can be used for active cooling (forproducing low temperatures or for dissipating heat).

3. Boosted thermal conductivity which can serve for efficient passivecooling for example in air conditioning installations or for (power)electronics.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the invention will be apparent byreference to FIGS. 1 through 5 and the related specific description. Inparticular:

FIG. 1 shows a thermoelectric element of a first embodiment;

FIG. 2 shows a thermoelectric element of a second embodiment;

FIG. 3 shows a thermoelectric element of a third embodiment;

FIG. 4 shows a thermoelectric element of a fourth embodiment;

FIG. 5 shows a thermoelectric element of a fifth embodiment; and

FIGS. 6-10 show a thermoelectric device illustrating the state of theart.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a thermoelectric element with a thermopair which has afirst material 1 with a positive Seebeck coefficient and a secondmaterial 2 with a negative Seebeck coefficient. A pn-junction 3 isfurther provided in a diode 8. The first material 1 is contactedselectively by way of an electrical conductor 6 to the p-side 4 of thepn-junction 3. The second material 2 is contacted selectively by way ofthe electrical conductor 7 to the n-side 5 of the pn-junction 3.

When the poles of an external voltage source (not shown) are connectedto the contacts 11, 12, the illustrated thermoelectric element operatesas a thermoelectric cooler. In that case the pn-junction 3 iselectrically poled in the reverse direction by the external voltagesource, in regard to one of the two possible polarities. In that casethe one end (T_(cold)) of the materials 1, 2 cools down. The other end(T_(hot)) of the materials 1, 2 heats up.

If in contrast the thermoelectric element is used as a thermoelectricgenerator, then the one end (T_(cold)) of the materials 1, 2 iscontacted with a heat sink (not shown) while the other end (T_(hot)) ofthe materials 1, 2 is contacted with a heat source. That produces in therespective materials 1, 2 an electrical voltage (in the first material 1the side towards the electrical conductor 6 is negatively charged up; inthe second material 2 the side towards the electrical conductor 7 ispositively charged up). That causes the pn-junction 3 to be connected inthe reverse direction. In that case the electrical field produced in thepn-junction 3 spatially separates generated electron-hole pairs whichcan therefore appear as a nett current. A voltage can be taken off byway of the contacts 11, 12, with the contact 11 being the negative poleand the contact 12 the positive pole.

A further embodiment of the invention is shown in FIG. 2. In thisembodiment the pn-junction 3 is provided in an intermediate layer 13.The pn-junction 3 as well as the first material 1 and the secondmaterial 2 (here p-doped and n-doped semiconductor respectively) arethermally connected to a heat source 9. The first material 1 and thesecond material 2 are further thermally connected at their other end toa heat sink 10.

The first material 1 is contacted by way of the electrical conductor 6to the p-side 4 of the pn-junction 3. The second material 2 is contactedby way of the electrical conductor 7 with the n-side 5 of thepn-junction 3.

An anode 12 and a cathode 11 are provided at the respective other end ofthe materials 1, 2, by way of which in the illustrated case of use ofthe thermoelectric element as a thermoelectric generator a voltage canbe taken off.

In the embodiment of the invention shown in FIG. 3 the pn-junction 3 isarranged spatially separate from the thermopair and connected theretoonly by way of electrical conductors 61 through 63 and 71 through 73respectively.

The diode 8 is connected in that case to separate heat sources 14, 15while the thermopair is thermally contacted with its own heat source 9.The thermopair is also in thermal contact with a heat sink 10.

A similarly constructed thermoelectric element is shown in FIG. 4. Theembodiment shown in FIG. 5 differs from that of FIG. 4 in that on theone hand there is a common heat source 9 and on the other hand there isan insulating layer 16 by virtue of the spatially compact configuration.

FIG. 6 shows a thermoelectric generator according to the state of theart, wherein the first material 1 and the second material 2 areconnected together in the region of the heat source 9 by way of anelectrical conductor 17. An anode 12 and a cathode 11 are provided inthe region of the heat sink 10. An electric current I flows due to thetemperature difference.

FIGS. 7 through 10 show further state of the art, namely on the one handa view showing the principle of a thermoelectric module (FIG. 7) made upof individual thermoelectric elements as shown in FIG. 6. Thediagrammatic view in principle of a thermoionic converter is shown inFIG. 8. FIG. 9 shows a thermal diode. FIG. 10 shows a thermoelectricelement whose pn-junction 3 has a temperature gradient.

The same references denote the same components throughout the entirespecific description.

1. A thermoelectric apparatus comprising: a thermoelectric elementincluding a thermopair and a pn-junction having a p-side and an n-side;a heat source; and a heat sink, wherein the thermoelectric element, theheat source, and the heat sink are operably coupled to form athermoelectric generator, wherein the thermopair includes a firstmaterial with a positive Seebeck coefficient and a second material witha negative Seebeck coefficient, wherein the first material isselectively contacted by way of a first conductor with the p-side of thepn-junction and the second material is selectively contacted by way of asecond conductor with the n-side of the pn-junction, wherein the heatsource is thermally coupled to a hot end of the thermopair and the heatsink is thermally coupled to a cold end of the thermopair, and the heatsource and the heat sink create a temperature difference in thethermopair between the hot end and the cold end of the thermopair whichgenerates thermoelectric voltage, wherein the thermoelectric voltageelectrically poles the pn-junction in the reverse direction by way ofthe first and second conductors, and wherein the first conductor isphysically connected to the heat source and the p-side of thepn-junction, and the second conductor is physically connected to theheat source and the n-side of the pn-junction.
 2. The thermoelectricgenerator of claim 1, wherein a side of the first material at the hotend of the thermopair is electrically connected to the p-side of thepn-junction, and a side of the second material at the hot end of thethermopair is electrically connected to the n-side of the pn-junction.3. The thermoelectric generator of claim 1, wherein the thermopair isarranged such that a side of the first material at the hot end of thethermopair is directly adjacent to a side of the second material at thehot end of the thermopair, and a side of the first material at the coldend of the thermopair is directly adjacent to a side of the secondmaterial at the cold end of the thermopair.
 4. The thermoelectricapparatus of claim 1, wherein the first material is a p-dopedsemiconductor.
 5. The thermoelectric apparatus of claim 1, wherein thesecond material is an n-doped semiconductor.
 6. The thermoelectricapparatus of claim 1, wherein the pn-junction is provided in a diode,and wherein the thermopair and the pn-junction comprise differentmaterials.
 7. The thermoelectric apparatus of claim 1, wherein thethermopair and the pn-junction are arranged spatially separately fromeach other.
 8. The thermoelectric apparatus of claim 1, wherein thethermopair and the pn-junction are connected together only by the firstand second conductors.
 9. The thermoelectric apparatus of claim 1,wherein the pn-junction is of a textured structure to increase theinternal surface area.
 10. The thermoelectric apparatus of claim 1,wherein the pn-junction is doped.
 11. The thermoelectric apparatus ofclaim 1, wherein the pn-junction has crystal defects.
 12. Thethermoelectric apparatus of claim 1, wherein the cross-section of thethermopair is different from the cross-section of the pn-junction. 13.The thermoelectric apparatus of claim 1, wherein the thermoelectricgenerator is configured and arranged such that the temperature of thepn-junction (Tpn) is different from the temperature (Thot) of the hotside of the thermopair.
 14. The thermoelectric apparatus of claim 1,wherein the thermopair is connected to the heat sink.
 15. Thethermoelectric apparatus of claim 1, wherein the first material isconnected to a first contact and the second material is connected to asecond contact, and wherein the first material is spatially separatedfrom the heat sink by the first contact and the second material isspatially separated from the heat sink by the second contact.
 16. Athermoelectric apparatus comprising: a thermoelectric element includinga thermopair and a pn-junction having a p-side and an n-side; a heatsource; and a heat sink, wherein the thermoelectric element, the heatsource, and the heat sink are operably coupled to form a thermoelectricgenerator, wherein the thermopair includes a first material with apositive Seebeck coefficient and a second material with a negativeSeebeck coefficient, wherein the first material is selectively contactedby way of a first conductor with the p-side of the pn-junction and thesecond material is selectively contacted by way of a second conductorwith the n-side of the pn-junction, wherein the heat source is thermallycoupled to a hot end of the thermopair and the heat sink is thermallycoupled to a cold end of the thermopair, and the heat source and theheat sink create a temperature difference in the thermopair between thehot end and the cold end of the thermopair which generatesthermoelectric voltage, wherein the thermoelectric voltage electricallypoles the pn-junction in the reverse direction by way of the first andsecond conductors, and wherein the heat source includes a first heatsource and a second heat source, the first heat source being spatiallyseparated from the p-side of the pn-junction by the first conductor, andthe second heat source being spatially separated from the n-side of thepn-junction by the second conductor.
 17. The thermoelectric apparatus ofclaim 16, wherein the first material is connected to a first contact andthe second material is connected to a second contact, and wherein thefirst material is spatially separated from the heat sink by the firstcontact and the second material is spatially separated from the heatsink by the second contact.
 18. A thermoelectric apparatus comprising: athermoelectric element including a thermopair and a pn-junction having ap-side and an n-side; a heat source; and a heat sink, wherein thethermoelectric element, the heat source, and the heat sink are operablycoupled to form a thermoelectric generator, wherein the thermopairincludes a first material with a positive Seebeck coefficient and asecond material with a negative Seebeck coefficient, wherein the firstmaterial is selectively contacted by way of a first conductor with thep-side of the pn-junction and the second material is selectivelycontacted by way of a second conductor with the n-side of thepn-junction, wherein the heat source is thermally coupled to a hot endof the thermopair and the heat sink is thermally coupled to a cold endof the thermopair, and the heat source and the heat sink create atemperature difference in the thermopair between the hot end and thecold end of the thermopair which generates thermoelectric voltage,wherein the thermoelectric voltage electrically poles the pn-junction inthe reverse direction by way of the first and second conductors, whereinthe heat source includes a first heat source, a second heat source, anda third heat source, and wherein the first heat source is spatiallyseparated from the p-side of the pn-junction by the first conductor, thesecond heat is being spatially separated from the n-side of thepn-junction by the second conductor, and the third heat source isconnected to each of said first conductor and said second conductor. 19.The thermoelectric apparatus of claim 18, wherein the first material isconnected to a first contact and the second material is connected to asecond contact, and wherein the first material is spatially separatedfrom the heat sink by the first contact and the second material isspatially separated from the heat sink by the second contact.
 20. Thethermoelectric apparatus of claim 1, wherein the heat source includes afirst heat source and a second heat source, the first heat source beingspatially separated from the p-side of the pn-junction by the firstconductor, and the second heat source being spatially separated from then-side of the pn-junction by the second conductor.